This invention relates to the field of semiconductor devices and methods of making same, and, more particularly, to a semiconductor device having a layer of quantum dots and at least one other layer, spaced therefrom, which communicates carriers in either direction with the quantum dot layer, and to a method of making such a device.
In 1977 there was introduced the quantum-well (QW) laser, a laser that in the active region employs one or more thin quantum well layers (Lz less than about 400 xc3x85) sandwiched in a thicker waveguide region that is further enclosed by still higher gap p-type and n-type confining layers (see E. A. Rezek, N. Holonyak, Jr., B. A. Vojak, G. E. Stillman, J. A. Rossi, D. L. Keune, and J. D. Fairing, xe2x80x9cLPE In1xe2x88x92xGaxP1xe2x88x92zAsz (xxcx9c0.12, zxcx9c0.26) DH Laser With Multiple Thin-Layer ( less than 500 xc3x85) Active Region,xe2x80x9d Appl. Phys. Lett., vol 31, pp 288-290, Aug. 15, 1977; E. A. Rezek, H. Shichijo, B. A. Vojak, and N. Holonyak, Jr., xe2x80x9cConfined-Carrier Luminescence of a Thin In1xe2x88x92xGaxP1xe2x88x92zAsz Well (xxcx9c0.13, zxcx9c0.29; xcx9c400 xc3x85) in an InP p-n Junction,xe2x80x9d Appl. Phys. Lett., vol. 31 pp. 534-536, Oct. 15, 1977). For about ten years, numerous groups have been trying to convert the quantum well laser, which is practical and widely manufactured, into a so-called quantum-dot laser. The goal has been to build an ultimate laser. In theory, the thin quantum well layer (or layers) of a QW laser is xe2x80x9ccut-upxe2x80x9d into (ideally) a uniform dense sheet of identical little quantum boxes (i.e., quantum dots). With such a structure, instead of the electrons and holes being injected (via current, light, etc.) and collected in an easily grown thin (Lz less than about 400 xc3x85) uniform QW sheet (thus with one dimensional thickness confinement and quantization; a 2-D structure), the electrons and holes are collected and recombine in the little quantum dots (i.e., quantum boxes; hence, three-dimensional confinement and quantization; a 0-D structure). The little boxes, or dots, are like xe2x80x9cfatxe2x80x9d little molecules and (ideally) all alike and densely and uniformly arranged. [Reference can be made, for example, to U.S. Pat. No. 6,369,403, and to J. H. Ryou, R. Dupuis, G. Walter, D. Kellog, N. Holonyak, Jr., D. Mathes, R. Hull, C. Reddy, and V. Narayanamurti, Appl. Phys. Lett. 78, 4091 (2001).] Actually, the quantum dots are stochastic and probabilistic, small but random. They are random in geometry, in size, and in arrangement - - - and not dense enough, i.e., are not in sufficient proximity to be quantum coupled (by tunneling). Electron-hole (e-h) pairs are stuck in each box (assuming they are collected at all) and don""t transport to neighboring boxes. The e-h pairs don""t move around and re-arrange in the plane of the dots as readily as they do in a thin QW sheet (Lz less than about 400 xc3x85; Lx, Ly extended in the plane; hence a 2-D structure). In short, there is little or no conduction along the sheet (or sheets) of quantum dots.
It is among the objects of the present invention to provide devices, and methods of making same, which are responsive to the foregoing limitations of the prior art, and to also provide devices which exhibit improved operation as light emitters and as other applications.
A major problem with quantum dot heterostructures (QDHs) is the stochastic nature of QD structures, the lack of uniformity, insufficient QD density, and poor QD-to-QD coupling. In accordance with an aspect hereof, a solution to the problem is to couple, via a thin barrier, the sheet (or multiple sheets) of QDs to an auxiliary layer. In a form hereof, the auxiliary layer is a QW layer, e.g., the QW layer of a QW laser. The higher energy states of the QDs are resonant with and couple by tunneling to the lower confined-particle states of the thin uniform QW layer. Electron-hole transport in the QW layer can re-arrange the e-h pairs in the plane and provide the dot-to-dot coupling. That is, an electron tunnels from the QD to the QW layer, transports in the plane in the QW layer, and re-tunnels back into another QD. Hence, one can couple the QDs - - - not in the plane, but out of the plane into a QW layer, then at some other point in the plane of the QW layer back into another QD. In this manner, the invention makes it possible for the charge to get unstuck from a QD and move from dot to dot in the waveguide region to help optimize emission.
Also, should the QDs not collect injected electron-hole pairs efficiently (a distinct possibility), the thin auxiliary QW layer (or, if necessary or desirable, multiple QW layers) will collect the injected carriers and feed them via resonant tunneling into the quantum dots to then scatter the carriers down to the lower energy dot states for recombination (for photon generation and laser operation). It is clear that multiple QD layers and multiple QW auxiliary or connection (communication) layers can be employed and can be part of laser or other device designs.
In another form hereof, the auxiliary layer is a delta doped layer. [As known in the art, a delta doped layer is an extremely thin (up to several atomic layers) layer highly populated with dopant (at least about 1018/cm8).] As seen hereinbelow, the performance of a quantum dot device exhibits great improvement when delta doped auxiliary layers are introduced in the layers adjacent the quantum dots.
In accordance with a further aspect hereof, the quantum dots, since they are a lower gap component of the III-V QD heterostructure, can be doped or be left undoped. In other words, the QD can be modified into a quantum xe2x80x9cdoping ball.xe2x80x9d There may be advantages in employing n-type or p-type QD xe2x80x9cdoping ballsxe2x80x9d, just as there may be device-design reasons to leave the QW auxiliary connection or communication layer undoped, or doped n-type or doped p-type. The QW auxiliary connection layer can be close coupled to the QDs (small separating barrier thickness) or can be weakly coupled (large barrier thickness). Also, the QW auxiliary layer can be thicker and drop its lower confined-particle states below the lowest QD states, or can be made thinner and raise its lowest confined-particle states above the lowest QD states. The foregoing types of techniques and structures can be incorporated into field effect transistor devices. For example, doped QDs, i.e., xe2x80x9cdoping balls,xe2x80x9d can be used to modify the so-called pseudomorphic transistor into a new form of field effect transistor. The doped QDs can be employed with or without auxiliary QW layer(s).
After GaAs (Eg=1.42 eV) it becomes almost impossible to make a higher gap, say GaP (Eg=2.26 eV), tunnel diode. Higher gaps (higher energies) require narrower tunneling distances, which in a p-n tunnel diode requires higher doping, not, as is common, the lower impurity solubility that comes with higher gap.
A tunnel diode can be devised that is not one large uniform junction, but rather an array of microscopic tunnel junctions, for instance an array of doped quantum dots on the n side of a p-n junction. For example, the GaAs(p+)-InGaAs(n+)-GaAs(n+) p-n tunnel diode made in 1993 (Richard, et al., Appl. Phys. Lett. 63, 3616 (Dec. 27, 1993), i.e., the narrow gap InGaAs center region, a reduced barrier, can be xe2x80x9ccut upxe2x80x9d into quantum dots. The significance of this kind of structure for tunnel contacts on VCSEL lasers, etc., can be understood in conjunction with U.S. Pat. No. 6,369,403.
Similar doped QDs, of a type that are employed in a tunnel contact or a tunnel diode, are the same type of doped QDs, or doping balls (DBs) that can be used as doping sources in any layered heterostructure, including in all the various kinds of quantum dot (QD) light emitting devices (e.g. lasers) that are described in numerous articles in the journal literature. Doping dots (DDs), or doping balls (DBs), can be laced through high gap material (in layers) to act as the source of carriers. The little QD doping ball, being small enough, can be incorporated into the higher gap material even if lattice mismatched and not create defects (dislocations). The doping ball, and its xe2x80x9csqueezedxe2x80x9d higher energy states, carries impurity at the solubility limits of the QD material, for example, an InP QD in higher gap In0.5Ga0.5P or in In0.5Al0.3Ga0.2P, and not at the solubility limits of the higher gap layer (the matrix) into which the QDs are incorporated.
The layers or regions of doping QDs (DDs or DBs) are versatile layers (n-type, p-type, or undoped) that can be incorporated into heterostructure devices, just as quantum wells, barriers, coupling barriers, waveguide regions, confining layers, etc. are incorporated into devices. Various kinds of devices, light emitters (LEDs and lasers), detectors, transistors, etc., can benefit. The QDs needn""t be viewed as just little xe2x80x9cboxesxe2x80x9d for carrier recombination (as in today""s QD lasers), i.e., a xe2x80x9ccenterxe2x80x9d for improved carrier recombination, but, more broadly, the QD can be viewed as a special little xe2x80x9cboxxe2x80x9d that can carry impurity into a region that otherwise is difficult to dope, or that offers an opportunity for enhanced doping. All the III-V materials are candidates for QD layering and doping dot techniques and devices. The nitride materials are relatively high gap and relatively hard to dope, and would therefore particularly benefit from employment of doping dots.
In accordance with the invention, a method is set forth of forming a semiconductor device, comprising the following steps: providing a plurality of semiconductor layers; providing a means for coupling signals to and/or from layers of the device (for example, providing conductive layers and/or electrodes to which signals can be applied, or providing a suitable optical technique for excitation+, etc.); providing a layer of quantum dots disposed between adjacent layers of the device; and providing an auxiliary layer disposed in one of the adjacent layers, and spaced from the layer of quantum dots, the auxiliary layer being operative to communicate carriers with said layer of quantum dots. In one embodiment, the auxiliary layer comprises a quantum well, and in another embodiment the auxiliary layer comprises a delta doped layer. The delta doped layer is preferably doped to a concentration of at least about N=1018/cm3.
In accordance with a form of the invention, there is set forth a method of forming a semiconductor device, including the steps of: providing a plurality of semiconductor layers; providing a means for coupling signals to and/or from layers of the device; providing a quantum well disposed between adjacent layers of the device; and providing a layer of quantum dots disposed in one of the adjacent layers, and spaced from the quantum well, whereby carriers can tunnel in either direction between the quantum well and the quantum dots. In an embodiment of this form of the invention the quantum dot layer is spaced from said quantum well layer by a barrier layer and the barrier layer has a thickness in the range about 5 Angstroms to 100 Angstroms. In this embodiment, the step of providing a layer of quantum dots comprises providing doped quantum dots. Also in this embodiment, the step of providing doped dots comprises providing dots doped to a concentration of at least N=1015/cm3 for n-type doping, and providing dots doped to a concentration of at least N=1016/cm3 for p-type doping. Further in this embodiment, the step of providing dots comprises providing dots having an average size in the range 10 to 100 Angstroms height and 10 to 200 Angstroms diameter, and having an average density in the range 1010 to 1012 per cm2.
In accordance with a further form of the invention, there is provided a semiconductor device which comprises: a plurality of semiconductor layers; means for coupling signals to and/or from layers of the device; a quantum well disposed between adjacent layers of the device; and a layer of quantum dots disposed in one of the adjacent layers, and spaced from the quantum well, whereby carriers can tunnel in either direction between the quantum well and the quantum dots.
In accordance with another form of the invention, there is provided a semiconductor device which comprises: a plurality of III-V semiconductor layers; means for coupling signals to and/or from layers of the device; and a layer of doped quantum dots disposed in at least one of the layers as a source of carriers that can communicate in either direction between the dots and another layer.
Further features and advantages of the invention will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings.